Thanks for posting this in the forums! I'm apologize for the inconvenience of not being able to take advantage of the upgrade program. If you have not used the serial number to use the upgrade program before, it should work.
If it is not working, I would recommend contacting Sling Media Support. You may find a resolution by doing so.
Below is a link to an article that explains how you can contact us:
Have a great day!
The Sling Moderation Team